Automotive manufacturers and the National Highway Transportation Safety Association are investigating methods to disable vehicle air bags in situations where they may cause more harm than good. Typically, airbags have been developed to deploy with enough force to restrain a 175 lb. adult in a high velocity crash. Deployment of the same air bags when children are seat occupants may cause serious injury due to the force generated upon inflation of the bag.
As a result, seat weight sensors and systems are being developed in an attempt to determine when the passenger seat occupant is a child. Such systems should identify when the occupant is small, or even when a child is in a rear facing infant seat, a forward facing child seat or a booster seat. Occupant weight measurement when a child seat is present is further complicated by the downward force applied to the child seat by the tension of a seat belt. When a child seat is strapped tightly, the seat belt forces the child seat into the vehicle seat and can bag deployment when children or infants are present in the seat.
A variety of methods have been used for seat belt tension measurement. Copending U.S. Provisional Application Ser. No. 60/067,071 entitled "Villari Effect Seat Belt Tension Sensor", and copending U.S. Provisional Application Ser. No. 60/070,319 entitled "Compressive Villari Effect Seatbelt Tension Sensor", both assigned to the assignee of the instant invention, disclose two seat belt tension measurement systems utilizing sensors that operate on the principle known as the Villari effect. The Villari effect refers to the tendency of certain materials with magnetostrictive properties to inhibit or enhance the strength of an electromagnetic field within the material when the material is being subjected to compression or tensile stress. By measuring the field strength in magnetostrictive material placed in line with a seat belt mechanism, for example in a seat belt latch or a seat belt retractor, the relative tension in the belt may be calculated.
Furthermore, belt deflection techniques which guide a seat belt through a mechanical system that forces the belt out of a straight line when there is low tension have been used. Under high tension the seat belt forces the displacement of a mechanical deflector. This force may then be sensed utilizing an electromechanical switch. Tension measurement mechanisms have also been incorporated in the buckle of the seat belt. In one embodiment, a sliding buckle is biased back with a spring. When the belt is under heavy tension, the buckle pulls forward to control a switch that provides feedback to a vehicle processor.
The aforementioned seat belt tension measurement methods suffer from a number of disadvantages. Initially, a great number of additional parts are required for seat belt retractors or buckle configurations. This adds complexity (and therefore cost) to vehicle assembly and provides for considerable difficulty in retrofitting existing vehicles. Additionally, several of the aforementioned tension systems provide only a threshold level of tension detection.
The present invention may be used to detect whether the seat belt is under high tension thereby denoting that an infant seat is present. Furthermore, significant tension in the belt can be predicted without resorting to the complex instrumentation required to measure actual belt tension. Known belt tension measurement systems that directly contact the seat belt require additional hardware and sensors that increase component count and vehicle assembly complexity.